Matthew 5:27–28: Ἠκούσατε ὅτι ἐρρέθη· οὐ μοιχεύσεις. ἐγὼ δὲ λέγω ὑμῖν ὅτι πᾶς ὁ βλέπων γυναῖκα πρὸς τὸ ἐπιθυμῆσαι αὐτὴν ἤδη ἐμοίχευσεν αὐτὴν ἐν τῇ καρδίᾳ αὐτοῦ.
“You heard it was said, ‘Do not commit adultery,’ but I say to you that everyone who looks at a woman in order to covet her has already committed adultery with her in his heart.”
I'm not sure how this concept of "adultery in one's heart" is usually interpreted. I can see two possible interpretations:
Lusting or coveting after a women is wrong, no matter the context. The verses right after the ones I quoted are the very ones about casting off your eye if it causes you to stumble, which makes the point very clear (However this raises an interesting point with regards to sexual repression.)
Sexual desire is human nature and is only forbidden if a man intends to have inappropriate sexual relations with the woman. Jesus could not possibly tell men to ignore their sexual 'hunger' which is as natural as hunger for food.
I think a major point in finding the correct interpretation is to consider the historical context. Were most of Jesus' disciples married, or was it custom back them for men to marry as soon as they are of a sexually ripe age? Can someone help me find a sound interpretation?